1. Field of the Invention
This invention generally relates to systems and methods for reticle inspection. Certain embodiments relate to systems and methods for inspecting reticles using aerial imaging at off-stepper wavelengths.
2. Description of the Related Art
Semiconductor fabrication processes typically involve a number of lithography steps to form various features and multiple levels of a semiconductor device. Lithography involves transferring a pattern to a resist formed on a semiconductor substrate, which may be commonly referred to as a wafer. A reticle or a mask may be disposed above the resist and may have substantially transparent regions and substantially opaque regions configured in a pattern that may be transferred to the resist. As such, substantially opaque regions of the reticle may protect underlying regions of the resist from exposure to an energy source. The resist may, therefore, be patterned by selectively exposing regions of the resist to an energy source such as ultraviolet light, a beam of electrons, or an x-ray source. The patterned resist may then be used to mask underlying layers in subsequent semiconductor fabrication processes such as ion implantation and etch. Therefore, a resist may substantially inhibit an underlying layer such as a dielectric material or the semiconductor substrate from implantation of ions or removal by etch.
There are several types of reticles that are commercially available. For example, a reticle may be either a clear-field reticle or a dark-field reticle. A clear-field reticle has field or background areas that are opaque, and a dark-field reticle has field or background areas that are transparent. In addition, a binary reticle is a reticle having a patterned area that is either transparent or opaque. Binary reticles are different from phase-shift masks (PSM) that may include films that only partially transmit light, and these reticles may be commonly referred to as halftone or embedded reticles. If a phase-shifting material is placed on alternating clear spaces of a reticle, the reticle is referred to as an alternating PSM, an ALT PSM, or even a Levenson PSM. If a phase-shifting material is applied to arbitrary layout patterns, the reticle is referred to as an attenuated or halftone PSM, which may be fabricated by replacing the opaque material with a partially transmissive or “halftone” film. A ternary attenuated PSM is an attenuated PSM that includes OPC features. Each of the reticles described above may also include a pellicle, which is an optically transparent membrane that seals off the reticle surface from airborne particulates and other forms of contamination.
A process for manufacturing a reticle is similar to a wafer patterning process. For example, the goal of reticle manufacturing is forming a pattern in an opaque material such as a relatively thin chrome layer on a substantially transparent substrate such as glass. In addition, other appropriate opaque materials that may be used for reticle manufacturing include, but are not limited to, chromium, chromium oxide, and chromium nitride. Appropriate thicknesses for chrome layers may be approximately 1000 Å and may be deposited upon a glass substrate by sputtering. Additional appropriate transparent materials that may be used for reticle manufacturing include borosilicate glass or fused-silica (SiO2, “quartz”), which have good dimensional stability and transmission properties for wavelengths of exposure systems. Additional materials may also be used for reticle manufacturing. For example, a film underlying an opaque material may act as an adhesion layer. Such an adhesion layer may include, for example, a mixture of chromium, nitrogen, and oxide. In addition, a film formed on top of the opaque material may act as an anti-reflective layer. An appropriate anti-reflective layer may be formed of, for example, a relatively thin layer of Cr2O3.
Reticle manufacturing may include a number of different steps such as pattern generation, which may include moving a glass substrate having a chrome layer and a resist layer formed thereon under a light source as shutters are moved and opened to allow precisely shaped patterns of light to shine onto the resist thereby creating the desired pattern. Since the patterns generated by an integrated circuit designer for each level are generally polygons, these patterns are decomposed into rectangles. The reticle pattern is transferred to the resist-covered reticle blank by a step-and-repeat process to create a master plate. The master plate is used to create multiple working reticle plates in a contact printer. The contact printer brings the master into contact with a resist-covered reticle blank and has an ultraviolet light source for transferring the image to the resist on the reticle blank.
Alternatively, reticles may be made with lasers or e-beam direct write exposure. Laser exposure allows the use of standard optical resists and is faster than e-beam direct write exposure. In addition, laser systems are also less expensive to purchase and operate. Direct write laser sources are turned on and off with an acousto-optical modulator (AOM). An example of a commercially available direct write laser system is the ALTA 3000® laser writer available from ETEC Systems, Inc., Hayward, Calif. Direct write e-beam systems are often used to manufacture complex reticles since they produce finer line resolution than laser systems. In addition, direct write e-beam systems can also write larger die sizes than laser systems. Examples of commercially available direct write e-beam systems include the MEBES 4500 and 5000 systems available from ETEC Systems, Inc.
After the exposure steps, the reticle is processed through development, inspection, etch, strip, and inspection steps to transfer the pattern into the opaque material. Defects in reticles are a source of yield reduction in integrated circuit manufacturing. Therefore, inspection of a reticle is a critical step in the reticle manufacturing process. As minimum pattern sizes shrink and integrated circuits are designed with higher device densities, defects that were once tolerable may no longer be acceptable. For example, a single defect may be repeated in each die in stepper systems and may kill every die in single-die reduction reticles. In addition, VLSI and ULSI-level integrated circuit manufacturing require substantially defect-free and dimensionally perfect reticles due to the critical dimension (CD) budget of such manufacturing. For example, the overall CD budget for such integrated circuits may be approximately 10% or better thereby resulting in a CD budget for a reticle with about a 4% error margin.
Defects may be a result of incorrect designing of the reticle pattern and/or flaws introduced into the patterns during the pattern generation process. Even if the design is correct, and the pattern generation process is performed satisfactorily, defects in the reticle may be generated by the reticle fabrication process as well as during subsequent processing and handling. In addition to the many potential causes of defects, there are also many different types of defects. For example, bubbles, scratches, pits, and fractures may be a result of a faulty raw glass substrate. Defects in the opaque material may include particulate inclusions in the material, pinholes or voids in the material surface, and invisible chemical anomalies such as nitrides or carbides that may lead to erratic local etching and undesired patterns. Defects such as voids in the resist layer may produce pinholes that may lead to chrome spots. In addition, localized characteristics in the resist may also produced variations in characteristics of the resist such as resist solubility across the reticle substrate. Particulate matter may also be introduced to the reticle during processing and/or handling of the reticle. Defects that may result in inoperative devices or which would cause a die to be rejected at final inspection are commonly referred to as “fatal” or “killer” defects, while other may be commonly referred to as “nonfatal” defects.